Slabs of North American Continent Are Layered Like Cake

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The continent of North America is not a single, thick, rigid
slab, but is instead more similar to a layer cake, with a section
of 3-billion-year-old rock sitting atop much newer material, a
new study that probes the depth of the continent finds.

"This is exciting because it is still a mystery how continents
grow," said study researcher Barbara Romanowicz, director of the
UC Berkeley Seismological Laboratory.

"We think that most of the North American
continent was constructed in the Archean (eon) in
several episodes, perhaps as long ago as 3 billion years, though
now, with the present regime of plate tectonics, not much new
continent is being formed," Romanowicz said.

How cratons form

The Earth's original continents started forming some 3 billion
years ago when the planet was much hotter and convection in the
mantle more vigorous, Romanowicz said. The continental
rocks rose to the surface and eventually formed the lithosphere,
Earth's hard outer layer that includes the planet's crust and a
portion of the upper mantle.

These old floating pieces of the lithosphere, called cratons,
apparently stopped growing about 2 billion years ago as the Earth
cooled, though within the last 500 million years, and perhaps for
as long as 1 billion years, the modern era of plate tectonics has
added new margins to the original cratons, slowly expanding the
continents.

One of those original continents is the North American craton,
located mostly in the Canadian part of North America.

The history of the Earth's oldest continental plates is vague,
because details of their interiors are hidden from geologists.
The deep interior of the North American craton is known only from
so-called xenoliths — rock inclusions in igneous rock (formed
from molten magma) — or xenocrysts such as diamonds that have
been delivered to the surface from deep below by volcanoes.

Seismologists, however, have the ability to probe the Earth's
interior thanks to seismic waves from earthquakes around the
globe, which can be used much like sound waves are used to probe
the interior of the human body.

Such seismic tomography has established that the bottom of the
North American craton is about 155 miles (250 kilometers) deep at
its thickest, thinning out toward the margins where new chunks
have been added to the continental lithosphere.

The new study suggests that any continental lithosphere that has
been added since the original North American craton formed came
from material scraped off of the ocean floor as the craton
plunged beneath the continent – and not deposited from below by
plumes of hot material welling up through the mantle, as happens
at volcanoes and mid-ocean ridges on the seafloor.

Layered continental cake

Romanowicz and UC Berkeley postdoctoral fellow Huaiyu Yuan found
the boundary between the old craton and the younger material
while using a new seismic technique to locate the boundary
between the lithosphere and asthenosphere, the softer material
below the lithosphere on which the continental and oceanic
plates ride .

Instead, they found a sharp boundary 93 miles (150 km) below the
surface, far too shallow to be the lithosphere-asthenosphere
boundary. The scientists think the sharp boundary is between two
types of lithosphere: the old craton and the younger material
that should match the chemical composition of the sea floor.
Their interpretation fits with studies of xenoliths and
xenocrysts, which indicate that there are two chemically distinct
layers within the Archean crust.

Another study conducted three years ago that also used seismic
waves to probe the Earth's deep layers found a sharp boundary at
a depth of about 75 miles (120 km).

"We think they are seeing the same layering we are seeing, a
sharp boundary within the lithosphere," Romanowicz said.

Romanowicz thinks their study will help scientists further tease
apart the formation of the continents.

"I think our paper will stimulate people to look more carefully
at distinguishing the ages of the lithosphere as a function of
depth," she said. "Any information we can provide that constrains
models of continental formation is really useful to the
geodynamicists."